1. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus and an ultrasound diagnosis program, and more in particular, to an ultrasound diagnosis apparatus and an ultrasound diagnosis program to synthesize images of a plurality of frames obtained by ultrasound scan.
2. Description of the Related Art
An ultrasound diagnosis apparatus is a device that ultrasound generated from a piezoelectric vibrator embedded in an ultrasound probe is radiated to a subject and a reflected signal generated by a difference of acoustic impedance of tissues of the subject is received by the piezoelectric vibrator for display. According to the ultrasound diagnosis apparatus, image data can be obtained easily in real time from simple manipulation performed by only touch of the ultrasound probe to a body surface. Therefore, the ultrasound diagnosis apparatus is widely used for diagnosis of a function of an internal organ and a shape thereof.
An ultrasound diagnosis method, which obtains information for a living body by a reflected wave from tissues of or blood cells of a subject, has been rapidly developed by two great techniques such as an ultrasound pulse reflecting method and an ultrasound Doppler method. B mode image data and color Doppler image data obtained by use of such techniques are indispensable for an ultrasound diagnosis these days.
Meanwhile, in a case of the ultrasound image data obtained by the ultrasound diagnosis apparatus, a particle type noise (so-called, a speckle noise) occurs by random interference of reflected ultrasound from a plurality of reflectors in a subject. Such speckle noise is known as an obstacle of the ultrasound diagnosis together with system noise of the ultrasound diagnosis apparatus.
As a method of reducing the speckle noise, there is, for example, a compound scan method. The compound scan method is divided into a spatial compound scan and a frequency compound scan.
The spatial compound scan is a technique that transmission and reception of ultrasound is performed for the same portion of a subject from a plurality of different directions, to sum and synthesize a plurality of obtained image data, thereby generating image data for display. On the other hand, the frequency compound scan is a technique that a plurality of image data is collected using different ultrasound frequencies for the same portion of a subject, to sum and synthesize the collected image data, thereby generating image data for display.
As a technique for a synthesis processing of image data as above, a following technique is disclosed in, for example, Japanese Unexamined Patent Application Publication No. H8-280688. In Japanese Unexamined Patent Application Publication No. H8-280688, a method of generating a composite ultrasound image including: dividing individually consecutive image frames into a plurality of sub-image areas to estimate and evaluate a local movement vector of the sub-image area; estimating and evaluating an overall image movement based on the estimated and evaluated local movement vector; and displaying a composite XFOV image based the estimated and evaluated overall image movement, is disclosed.
According to the method disclosed in Japanese Unexamined Patent Application Publication No. H8-280688, a large composite ultrasound image can be generated, which is displayed as an image of an enlarged viewing angle (XFOV).
In addition, as a method of reducing the speckle noise, a following technique is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2005-296331. That is, in Japanese Unexamined Patent Application Publication No. 2005-296331, there is disclosed a ultrasound diagnosis apparatus including: image data generating means for performing a transmission and reception of ultrasound from a plurality of directions of a subject to generate image data; wavelet transform means for performing a wavelet transform for the image data to calculate a low frequency wavelet transform coefficient and a plurality of high frequency wavelet transform coefficients for each resolution level; edge information detecting means for detecting edge information for the image data based on the high frequency wavelet transform coefficients; coefficient processing means for operating a weighting of the high frequency wavelet transform coefficients based on the detected edge information; wavelet inverse-transform means for generating edge-emphasized image data by a wavelet inverse-transform using the weighting-operated high frequency wavelet transform coefficients; and display means for displaying the edge-emphasized image data.
According to the ultrasound diagnosis device disclosed in Japanese Unexamined Patent Application Publication No. 2005-296331, it becomes possible to effectively reduce a speckle noise and to emphasize an edge in ultrasound image data.
Upon display of a tissue image with an ultrasound diagnosis apparatus, as synthesizing methods at the time of synthesizing and displaying image data obtained by an ultrasound scan, there are mainly three following methods known.
(Synthesizing Method 1) Spatial Compound Scan
In a case of the spatial compound scan, as shown in FIG. 8, an ultrasound probe 101 is fixed and held to perform a scan from different directions each frame. When the ultrasound scan is performed from different directions as above, and since interference conditions are different even in an echo from the same place, correlation with a speckle becomes less. Therefore, a mean value operation or a maximum value operation is performed for an overlap region (an overlap region 107 of a first scan image 103 and a second scan image 105), to reduce a speckle.
In addition, a tissue border which is vertical to an ultrasound beam generates a clearer border echo by specular reflection. With this, the irradiation of the ultrasound beam from multiple directions enables echoes from a tissue border at various angles to be displayed clearly.
(Synthesizing Method 2) Panoramic View
The panoramic view is an image method as such disclosed in Japanese Unexamined Patent Application Publication No. H8-280688, which is a technique for displaying a wider viewing angle that, as shown in FIG. 9, an ultrasound probe 101 moves towards a section of a scan and the movement of the ultrasound probe 101 is detected from the obtained image data, to adjust a position by translational movement and rotational movement of the image data for a synthesis processing. Currently, a synthesis processing of overwriting the latest image on a previous image is generally performed.
(Synthesizing Method 3) Combination Focus
The combination focus is a technique that, as shown in FIG. 10, transmission and reception of ultrasound beam is performed multiple times changing a transmission focus from the same direction not from different directions, to synthesize images obtained by the transmission and reception. That is, in the combination focus, an image 111A of a short-distance region is obtained by an ultrasound beam 111 of a short-distance focus and an image 113A of a long-distance region is obtained by an ultrasound beam 113 of a long-distance focus, to synthesize the obtained images.
However, the above-described methods of synthesizing image data have the respective problems as follows.
First, in the case of the spatial compound scan, upon synthesizing the image data, as shown in FIG. 8, an overlap region 107 and non-overlap regions 109 are generated between image data (the first scan image 103 and the second scan image 105) related to the corresponding synthesizing. In this case, for the non-overlap regions 109, the image data related to the corresponding synthesizing is displayed as it is. On the other hand, for the overlap region 107, an image synthesis processing is performed by an image synthesizing method such as a mean value operation or a maximum value operation or the like.
Herein, if employing the maximum value operation as the image synthesizing method, a border between the overlap region 107 and the non-overlap regions 109 is displayed naturally. However, when an artifact having a higher gray scale is present in one image data, the artifact is displayed as it is. Meanwhile, if employing the mean value operation as the image synthesizing method, the border between the overlap region 107 and the non-overlap regions 109 is displayed unnaturally. Even in any cases, that is, even if employing the maximum value operation or the mean value operation as the image synthesizing method, there is a problem in that the image data after the corresponding synthesis processing looks somewhat blurred to the viewer.
In addition, in the case of using the panoramic view, when the overlap region is generated, the overwriting processing is performed for the image data of the corresponding region as described above. Thus, a blurring of the image due to the synthesis processing does not occur. However, notwithstanding the presence of the image data in the overlap region, it cannot be said that unuse of the image data sufficiently enjoys a merit according to the synthesis processing of the image data. In other words, use of the image data in the overlap region can obtain the same effect as the above-described spatial compound scan. However, since it has the same problem as the spatial compound scan at the same time, the problem which the spatial compound scan has is required to be solved, whatever it takes.
In addition, like the use of the panoramic view for example, the image data obtained by the scan from different positions has a clear region and a blurred region in the corresponding image data due to a position where each image data is obtained.
For example, as shown in FIG. 11, for a first image 133 where a blurred region is present in the right of a region of interest 131 and a second image 135 where a blurred region is present in the left of the region of interest 131, the mean value operation or the maximum value operation is carried out to perform a synthesis processing, and this leads to obtaining only an image data where the blurred region is enlarged.
Therefore, in this case, it is preferable to obtain the image data 137 with the clear regions only by use of only the clear region for the synthesis processing, not by use of the blurred regions present in the image data related to the corresponding synthesis processing. However, currently, a technique for performing such synthesis processing automatically is not disclosed.
In the case of using the combination focus, and since the combination focus divides the image data for synthesizing, it has a problem that a border is visible on the image data after the corresponding synthesis processing. A method to sum weights varying a little is considered, but a synthesis processing method to contribute to increasing a resolving power is more preferable than the method to sum varying weights.
Of course, the techniques disclosed in Japanese Unexamined Patent Application Publication No. H8-280688 and Japanese Unexamined Patent Application Publication No. 2005-296331 do not solve the problems described with reference to FIGS. 8 to 11.